The present invention relates to a thermal transfer sheet for the formation of a color image. More particularly, it is concerned with a thermal transfer sheet for the formation of a high-sensitivity full-color image, which thermal transfer sheet can reproduce a wide range of colors comparable to various types of conventional color printing on a commercial scale and can provide images having excellent light fastness.
Color printing has hitherto been carried out in large volumes by offset printing, gravure printing or other printing methods. In such color printing, an original per se or a combination of an original with other originals, letters, symbols and the like is subjected to color separation to prepare plates for three colors of cyan, magenta and yellow. A plate for black is further prepared according to need, and the hue and pattern of the original is reproduced using printing inks for respective colors.
In most cases, pigments are used as colorants for inks of three primary colors of cyan, magenta and yellow in the above printing system. In this case, three primary color pigments known to be best suited for color printing from various experience in the past are selected so that a wide variety of colors ranging from the three primary colors to colors intermediate between them can be reproduced.
The above-described conventional printing system has a problem that high equipment cost and large space are necessary because it is indispensable to always prepare plates respectively for the three primary colors or plates respectively for four colors (i.e., black in addition to the three primary colors). For example, color printing cannot be simply carried out in small workshops and offices.
In order to solve the above problem, a proposal has been made on a thermal transfer system where a thermal transfer sheet for three primary colors is prepared using sublimable (or thermomigratory) dyes and the dyes are transferred by means of thermal energy using the thermal transfer sheet system requires neither a large printing machine nor other various equipment and enables color images to be simply formed, which renders the thermal transfer system promising.
In the above-described thermal transfer system, a material, to which an image is to be transferred, and a thermal transfer sheet are put on top of the other, and thermal energy is applied by a thermal head or other means from either the material, to which an image is to be transferred, or the thermal transfer sheet, thus causing dyes on the thermal transfer sheet to be transferred to the material to which an image is to be transferred. Color dots formed by the transfer have much larger size than dots (halftone dots) in the conventional offset printing. Further, in the case of printing inks, the color density of halftone dots can be changed as desired mainly by regulating the size of halftone dots. On the other hand, in the thermal transfer system, a change in dot size is difficult, and the density difference should be varied by varying thermal energy applied.
For this reason, it is ideal for the magnitude of thermal energy applied to be proportional to the density of dots to be transferred. In the conventional thermal transfer sheet, however, the migration of dyes, that is, the color density, is poor particularly when the thermal energy is low. Further, since it is practically impossible to regulate the migration of the three primary colors on the same level, the reproduction of a relatively light color is unsatisfactory and, at the same time, intermediate colors having satisfactory gradation cannot be formed.
Further, in the case of a full-color image formed by the above-described system, if a dye having a poor light fastness is used even for only one color, the light fastness of the full-color image as a whole is poor even when the other dyes used have an excellent light fastness. Although dyes having an excellent light fastness are individually known, a combination of dyes capable of providing an excellent full-color image through a combination of the three primary colors is not known in the art.
Further, in most cases, the three primary colors in the conventional offset printing inks comprise pigments. On the other hand, all the colorants used in the thermal transfer sheet comprise sublimable (or thermomigratory) dyes. Therefore, both systems are different from each other in color developing mechanism. This renders very difficult to select sublimable (or thermomigratory) dyes which match the three primary colors in the offset printing inks.
Furthermore, in the conventional dyes, when color images are formed using the three primary colors of cyan, magenta and yellow, it is very difficult to reproduce colors intermediate between these three colors. Thus, in order to provide a color image having a quality close to a printed image by the thermal transfer system, the development of a thermal transfer sheet capable of reproducing a wide variety of colors ranging from the three primary colors to colors intermediate between the three primary colors has been a very important technical task in the art.